He's very active on KTMtalk, one of the engine gurus there. You might want to join up there for everything technical and also for a really nice discount at Munn Racing if you get a lifetime membership.

He's very active on KTMtalk, one of the engine gurus there. You might want to join up there for everything technical and also for a really nice discount at Munn Racing if you get a lifetime membership.

Sean

Yeah I'm on that. Just ordered some stuff from Munn yeterday, best prices I could find on anything.
I'll look into the motor dude.
Gracias...

You know, I did the Athena 290 on my WR250X and it really made the bike alot more vibey. I'm reticent to do a big bore kit on this one.
I'd like to hear from someone who did this...

You know, I did the Athena 290 on my WR250X and it really made the bike alot more vibey. I'm reticent to do a big bore kit on this one.
I'd like to hear from someone who did this...

My buddy did it and it really woke the motor up mid to top. The beauty of the 400 is the docile bottom, it gets traction out of the hole, and you don't want to mess that up. The 453 mod builds better mid and a really good top end without sacrificing the awesome bottom. My Dave Hopkins built 540 was a 60hp mini monster that would go 185 km/h on pavement, but on gravel my 540 and my buddy's 453 were side by side at just under 160 km/h, with the excess power just being turned into roost. I'm 6'2", 225lbs so my 540 didn't beat me up too much but at 5'8" 175lbs, my buddy's 453 was an easier and smoother bike for him to handle and it made him a faster rider. Most people will try the bigger motors and like the boost, but then they go too big and find that the excess is just too much and they come back to the combo that worked the best. IMO if you start with a 450 or 525 that best bang combo is the 540 and if you start with a 400 it's the 453. Going bigger just makes you work harder and wears out parts faster.

I have not noticed any increase in vibes from any of the BB KTMs I have ridden. These are thumpers so it is normal for some vibes but the short stroke 400 crank is already smooth to begin with that I wouldn't worry too much about it. When Dave built my 540 he aligned, welded and balanced the crank and it was very nice. Unfortunately, that process makes the crank a solid non-rebuildable chunk so that is the con.

...yeah, that's why I wanted the 400. I figured it would be more power than the WR290 and alot less weight. I read alot about how smooth it was too.
That's a +++ for me.
The bike is very mild, and that's what I need to ride better/quicker.

Next up, what tires to buy???
I've tried
T63's - so-so
D606's - O.K. but washy-outie front
M21's - liked these the best
MT43 rear/MT21 front - very limited use before I sold the WR.

Problem is OEM is 140-80/18 rear and I don't want to make the too bigga rear tire mistake again...

DOT:
For racing I use Pirelli Scorpion XC fronts and Pirelli Scorpion Pro FIM. The fronts are great when new but I'm heavy and aggressive so the outside knobs tend to shred off on me before the tire has a chance to wear out. The Scorpion Pro FIM is the only Pirelli Scorpion rear that is DOT. It's predictable and durable. Come in 120 or 140 size. I run 140s but if you don't run gravel very much the 120 is better in the woods. I prefer the 140 on gravel because it lets the bike slide out a bit more before it bounces off the outside knobs.
For endurance rallies (the Dacre things in my sig, 800kms in one day) I'll run an Interco Terraflex on the back only because it can go flat and it doesn't make a lick of difference. I'll change a front flat on a timed rally but rather avoid doing a rear.

For non racing I have taken a liking to the MotoZ Tractionator S/T front and I/T rear. The S/T rear works great too but I just shred them too fast on high speed gravel. The fronts are soft terrain but wear nicely on the light bikes. The knobs have huge shoulders so they don't rip off on me but the tire does slide around a bit on gravel and deep sand under no throttle. The rears are tough and I think I will actually run a few Enduros with it to see how they hold up.
If you like the MT21 (I didn't) try the Kenda K760 front and rear. Better traction, better price. The fronts wash out on me because of the tread pattern (the knobs are all inline across the tire) but work well if you don't hammer corners too much.

Non DOT:
My tire of choice if I don't need a DOT rating is the Michelin S12 front and rear. They just work everywhere but will wear out fast on pavement or hardpack. This is an MX tire but they work so well in the woods.